Add a feature flag for the USB DAC mode, and gate LANG_* appropriately

This is also a necessary step for proper inclusion in the manual

Change-Id: I1c30d2399509827ef6f0095485659d42f582cff9

apps/features.txt

@@ -308,3 +308,7 @@ codec_aac_he
 #if defined(USE_LTO)
 lto
 #endif
+
+#if defined(USB_ENABLE_AUDIO)
+usbdac
+#endif

apps/lang/english.lang

@@ -10375,13 +10375,16 @@
   desc: in settings_menu
   user: core
   <source>
-    *: "USB-DAC"
+    *: none
+    usbdac: "USB-DAC"
   </source>
   <dest>
-    *: "USB-DAC"
+    *: none
+    usbdac: "USB-DAC"
   </dest>
   <voice>
-    *: "USB-DAC"
+    *: none
+    usbdac: "USB-DAC"
   </voice>
 </phrase>
 <phrase>
@@ -10389,13 +10392,16 @@
   desc: in settings_menu
   user: core
   <source>
-    *: "While In USB Charge-Only Mode"
+    *: none
+    usbdac: "While In USB Charge-Only Mode"
   </source>
   <dest>
-    *: "While In USB Charge-Only Mode"
+    *: none
+    usbdac: "While In USB Charge-Only Mode"
   </dest>
   <voice>
-    *: "While In USB Charge-Only Mode"
+    *: none
+    usbdac: "While In USB Charge-Only Mode"
   </voice>
 </phrase>
 <phrase>
@@ -10403,13 +10409,16 @@
   desc: in settings_menu
   user: core
   <source>
-    *: "While In USB Mass-Storage Mode"
+    *: none
+    usbdac: "While In USB Mass-Storage Mode"
   </source>
   <dest>
-    *: "While In USB Mass-Storage Mode"
+    *: none
+    usbdac: "While In USB Mass-Storage Mode"
   </dest>
   <voice>
-    *: "While In USB Mass-Storage Mode"
+    *: none
+    usbdac: "While In USB Mass-Storage Mode"
   </voice>
 </phrase>
 <phrase>
@@ -10417,13 +10426,16 @@
   desc: for splash
   user: core
   <source>
-    *: "USB-DAC Active"
+    *: none
+    usbdac: "USB-DAC Active"
   </source>
   <dest>
-    *: "USB-DAC Active"
+    *: none
+    usbdac: "USB-DAC Active"
   </dest>
   <voice>
-    *: "USB-DAC Active"
+    *: none
+    usbdac: "USB-DAC Active"
   </voice>
 </phrase>
 <phrase>